This invention relates to actuators and to energy dissipation devices therefor and, in particular, to energy dissipation valves for hydraulic actuators used for tiltable marine drive units.
Linear hydraulic actuators typically include a piston reciprocatingly received within a cylinder. A piston rod is connected to the piston and extends through one end of the cylinder at least. The piston is moved by supplying pressurized hydraulic fluid to the cylinder on one side of the piston, causing the piston to move in the opposite direction. These actuators may be used for many purposes including tilt actuators and/or trim actuators for tiltable marine drive units. Tilt actuators are used to tilt outboard motors or inboard/outboard drives located at the stern of the marine craft. Tilt actuators tilt the drive units downwardly to render the drive units operational and tilt the drive units upwardly when not in use. Trim cylinders are used to adjust the angle of the drive units for proper operation of the marine craft.
Such drive units have a leg which extends downwardly with a propeller located near the bottom thereof. The leg and propeller are located below the bottom of the marine craft and are susceptible to hitting underwater objects. Damage to the drive unit, the marine craft itself or both the drive unit and the marine craft may occur as a result of such collisions. Accordingly it is conventional to permit extension of the tilt actuators and trim actuators under shock loading as occurs during collisions was underwater objects. This is conventionally achieved by providing energy dissipation valves or impact valves for tilt actuators and/or trim actuators. These valves are typically located on the piston of such an actuator and may comprise a spring-loaded valve member located on the passageway through the piston. An impact event increases pressure in the hydraulic fluid sufficiently to move the valve member against the spring and allow hydraulic fluid to flow through the passageway from one side of the piston to the other. This allows the actuator to extend as a result of an impact event. The extension of the actuator allows the drive unit to dissipate energy by tilting upwardly and minimizes damage resulting from the impact.
However the configuration and specifications of such valves had to be tightly controlled in order to provide proper energy dissipation. Simply put, if the valves only release once the pressure is at too high a level, then the drive unit would encounter too much resistance to tilting in the event that the motor contacts an object, potentially resulting in damage to the motor or boat. On the other hand, if the valve releases at too low a pressure, the motor may swing upwardly too fast, again potentially damaging the motor or boat. The valve must dissipate sufficient energy with respect to the impact and the inertia of the drive to not cause significant damage to the marine craft or drive.
One of the critical factors with some earlier energy dissipation valves is the preload on the valve spring. The preload on the valve had to be tightly controlled in order for the valve to release at an appropriate pressure level and to dissipate the correct amount of energy during an impact event. In fact, prior art valves cope with high energy situations, for example when the motor encounters an object at high speed, by increasing the preload on the spring. However the effect of increasing preload is to lose low energy capability. In other words, the valve does not sufficiently open to dissipate energy during low speed collisions.
The invention is also applicable to other applications besides marine drives where it is desirable to dissipate energy by allowing movement of hydraulic actuators during shock or impact events. However the invention should be differentiated from standard shock absorbers, used on vehicles for example, which are active at all times and respond to hydraulic pressure fluctuations. The invention by contrast is adapted only to respond to extraordinary impact events. The valve does not responded to all impact events and resulting pressure increases below a certain threshold.